Acute lung injury (ALI) remains a major cause of morbidity and mortality. Patients with ALI require high oxygen for survival and hyperoxia contributes to cellular injury that may exacerbate recovery. Strong protective role of various cytokines and growth factors that signal through the signal transducers and activators of transcription3 (Stat-3) pathway has been shown previously. The role(s) and mechanisms by which Stat-3 influences pulmonary homeostasis are presently unknown. Our preliminary data demonstrated that cre-mediated deletion of Stat-3 in respiratory epithelial cells of the lung render the mice highly susceptible to hyperoxia. Surfactant homeostasis was disrupted in the Stat-3 deleted mice demonstrating a critical role for Stat-3 in signaling in the respiratory epithelium during oxygen induced injury. The present application is therefore designed to discern the mechanisms by which Stat-3 protects the lung during hyperoxic injury, Aim 1 will test the hypothesis that Stat-3 modulates cytoprotective responses to limit O2 induced damage, and/or Stat-3 is required for epithelial cell gene expression and proliferation required for repair following oxygen injury. Conditional Stat-3 mice (Stat-3 delta/delta mice) in which Stat-3 is selectively deleted in conducting airway (CCSP-rtTA) or peripheral airway epithelial cells (SP-C-rtTA) will be utilized for these studies. Aim 2 will test the hypothesis that epithelial cell Stat-3 is activated by ligand activation of gp130 dependent pathways. Stat-3 phosphorylation and cytoprotection during hyperoxia will be assessed in transgenic mice in which gp130 is deleted with cre-recombinase and respiratory epithelial cells. Aim 3 will test the hypothesis that Stat-3 regulates the expression of SP-B and other surfactant components during hyperoxia. The primary role of SP-B in the pathogenesis of hyperoxic lung injury in the Stat-3 delta/delta mice will be assessed after genetic and therapeutic replacement of SP-B in vivo. Aim 4 will test the hypothesis that Stat-3 directly regulates the transcription of subsets of genes critical to the maintenance of epithelial cell homeostasis. The proposed studies will determine the effects and mechanisms by which Stat-3 and its associated pathways mediate protection of the lung during acute lung injury and will identify the cellular processors and genes critical for repair of the lung. These pathways should be of considerable interest in the design of therapeutic strategies for the treatment or prevention of acute lung injury.